With Photoshop, there’s not much need to carry around a book of physical filters anymore, but there are still two that you have to make sure you have. Want to know why?
You can now duplicate just about any effect from a physical filter with a digital effect. Whether it’s exposure blending and HDR to replace a gradient neutral density filter, channel adjustments for controlling contrast in a black and white conversion, or special effects like soft focus, you can get away with just doing it in Photoshop. However, the results of using a strong neutral density filter or polarizer just can’t be replaced in PS yet.
Let’s talk about polarization first. A polarizing filter like the B+W Circular Polarizer allows only light polarized to a specific angle to pass through. To a photographer, this translates into darker skies, reduced reflections, and diminished glare. Often a good choice for landscapes, seascapes, or shots where there’s glare on foliage, a polarizer can breathe life into a variety of shots.
In that example shot, you can see the polarizer shot on the right has visual information that just doesn’t exist in the before shot. While you might be able to dial in a ton of added contrast to try and bring back those details, it’s just not going to be practical.
A polarizer can also be used to emphasize reflections and other sources of polarized light, making them more intense. This can make a big difference when photographing pools of water, rainbows, or skylines. Amping up the reflection via added polarization just looks more natural than adding tonal contrast digitally or will at least get you closer to the desired result before you have to lean on that contrast slider. One thing to keep in mind with polarizers is to avoid the older style of linear polarizer; these can interfere with autofocus systems but are less common these days.
The case for the neutral density filter is a little less watertight. By stacking a series of exposures and blending them, you can replicate the impact of a neutral density filter to a reasonable degree. I've used this technique many times, for everything from waterfalls to star trails, and it's been a fun trick to learn.
Where the filter makes the biggest difference is in visualization. When you’re shooting a stack of frames to blend, it’s very difficult to see if you’re actually getting the degree of movement that you want. In contrast, by using an actual ND filter, you’ll be able to see the results instantly on image review and can adjust right in the field. For this waterfall example, I first just tried the stack, as I didn't want to dig into the bottom of my bag while in the rain just to find the filter. After the first couple of shots, however, I realized I just couldn't tell how long I needed to get the degree of smoothness that I was looking for.
If you’re using a camera without a built-in intervalometer, an ND filter is even more important, as the blending technique doesn’t tolerate camera movement well, and stabbing at the shutter 30 times is sure to introduce some movement. Using an ND won't save you from the issue of camera movement, however, so make sure you're using a good tripod and head and have everything locked down. The difference it does make is the ability to click it and forget it, making it both easier to set up and shoot in comparison.
Additionally, some techniques just work better with an actual long exposure, like light painting. While you can paint individual elements and composite later, the larger movements and patterns will have gaps in them as the shutter cycles, and you again run into the visualization issue.
A neutral density filter is also a pretty good value proposition, especially when you consider the availability of both 10-stop and variable ND filters. A variable ND like Tiffen’s 77mm 2-to-8 stop offering can take the place of multiple ND filters, especially if you combine it with step-up rings to fit smaller filter sizes. I’m personally a fan of just buying one very strong ND filter, as I find that most of the time I’m using it, I need all the stops I can get. My personal choice is Breakthrough Photography’s 10-stop ND, which has had some of the best sharpness and color results in testing.
Everything isn't golden with neutral density filters, however. Two big downsides are expense and workflow impact. For a usefully dark filter (3+ stops of reduction), you'll be looking at about $100. Cheaping out will mean severe color casts and a loss of sharpness. The first ND filter I bought made everything magenta and certainly wasn't saving me any work in PS! Apparently, good dark glass is expensive, so I'd suggest taking the hit only once; buy the largest size you'll expect to need. You can easily convert down in filter sizes, but going too small will mean buying duplicates in bigger sizes down the road.
The second impact is on workflow. While this article is meant to explore how you can save post-processing time with some smart filter usage, ND filters are more of a trade-off rather than a straight improvement, at least at the beginning. There's a bit of a checklist for proper ND usage. You have to lock down focus before you put the filter on, as the heavier strengths are impossible to see or autofocus through. Changing compositions means screwing the filter on, grabbing the shot, realizing you included an errant tree branch, unscrewing to adjust, and repeating (I speak from experience here). You'll also have to adjust some camera settings, as well as close off your viewfinder, if possible.
In my opinion, neutral density filters and polarizers still have a place in my camera bag. While you can imitate some of the impact they have on a shot in post, these two stand apart from other filter types in just how much of a difference they can make in a photo.